This invention pertains to a method for lubricating metals and more particularly to the use of acrylic or methacrylic acid graft copolymers of poly(oxyalkylene) compounds neutralized with alkanolamines.
Fatty acids have historically been used as metalworking lubricants for forming, shaping, cutting, and grinding metals. They have also been used as co-lubricants and lubricant additives. These fatty acids containing one or more carboxyl groups per molecule have been derived from animal, vegetable and mineral sources. These fatty acids have been used as such in the acid form but are often neutralized with a base to make a soap. Common bases used for this purpose include alkali metal hydroxides and strong amines. These fatty acids may be saturated, e.g., stearic or lauric acids or they may be unsaturated, such as oleic or ricinoleic acids. Examples of soaps are delineated in Chemical Engineering, Volume 61, page 142, June 1954. Initially the fatty acid lubricants were used alone as frictional modifiers. As the art improved, the fatty acids were diluted with water and the resultant aqueous lubricants gave generally better performance, improved cooling and were lower in cost. In metalworking, the forming, shaping, cutting or grinding of metal generates considerable heat and the emphasis toward aqueous lubricants to obtain higher machining speeds at reduced costs occurred.
The next change to occur in the use of fatty acids lubricants was the combination of the acid with another lubricant. In aqueous lubricants the fatty acids, usually as the triethanolamine soap were combined with ethylene or propylene oxide polymers such as polyalkylene glycols. These combinations were particularly useful in chipless metalforming or shaping and to some extent in metal removing operations (cutting or grinding) where the liquid lubricant desirably keeps the metal surfaces separated. Good lubrication is obtained if the lubricant when subjected to high pressures between two surfaces continues to prevent direct contact of these surfaces. Therefore the capacity to withstand extreme pressures between surfaces is an important consideration in metalworking operations.
Water soluble copolymers of ethylene and propylene oxide have been most useful as co-lubricants with fatty acids for metalworking operations. These poly(alkylene) oxide polymers function because of their ability to deposit polymer onto the surfaces of the metal to be lubricated. This occurs because of the inverse solubility characteristics of these polymers. The heat and pressure at the point where lubrication is needed raises the solution's temperature above the polymer's cloud point. Therefore when polyalkylene glycol polymers and fatty acids have been used together, films are deposited which have greatly improved lubrication and antifunctional qualities. Aqueous solutions of fatty acids alone do not provide these advantages.
Polyalkylene glycol polymers have been combined with fatty acids in two ways for improved aqueous metalworking fluids. In the first method the components are simply mixed together with the fatty acids in the form of a triethanolamine salt in order to improve water solubility. In the second method poly(alkylene oxide) polymers have been combined with a fatty acid by esterification of the terminal hydroxyl group of the polymer with the carboxyl group of the fatty acid.
Despite the advantages of the prior poly(alkylene glycol) polymer/fatty acid lubricant combinations they suffer from several significant limitations. First of all amine salts of fatty acids are not stable in hard water. Stability in hard water is an important requirement since most industrial water used to dilute metalworking fluids is hard, containing calcium and/or magnesium cations in varying amounts. While inorganic salts of calcium or magnesium are generally soluble in water, organic salts are seldom soluble. Therefore, carboxylic acid salts which are water soluble as amine soaps, often precipitate from hard waters since they are not soluble as the calcium or magnesium soap. Inasmuch as lubrication and corrosion inhibition benefits are dependent upon a combination of two materials, preferential extraction or precipitation of one material is undesirable.
A second limitation of the above-described lubricant combinations is their tendency to foam excessively. This is a serious problem since the alkanolamine soaps of fatty acids can generate large amounts of foam which does not collapse, particularly in softer waters. This foaming tendency is a major barrier preventing the use of chelating agents to complex calcium and magnesium ions, since softening the water to improve fatty acid stability can often result in excessive foaming.
A third limitation of the above-described lubricant combinations is the difficulty of their preparation without the use of a neutral co-solvent to effect mixing of the two components. Any components added solely to provide a homogeneous mixture and do not contribute to performance characteristics obviously increase the cost of the commercial product. Conversely any reduction in the amount of co-solvent used is highly desirable.